Defining a 'cosmic haystack' in the search for aliens

An illustration of a radio-beam-powered light sail on an alien spacecraft.

Over the past 60 years, multiple SETI projects have looked and continue to look for alien signals. Some scan large swaths of the sky for powerful signals, while others target individual star systems for weaker signals.

Yet aside from a few anomaly signals that never repeated (like the "Wow!" detection of 1977), these searches have turned up empty-handed.

Kanodia and his colleagues at Penn State University wanted to know how much of the figurative "cosmic haystack" SETI projects had covered and to what extent they could improve the hunt for the alien "needle."

The group agrees with the well-known SETI astronomer Jill Tarter, who said in 2010 that it was silly to conclude intelligent aliens do not exist nearby just because we haven't yet found their beacons. Even if such signals exist and are aimed right at Earth, her thinking goes, we've scanned so little of the sky and may not be looking for the right type of signal, or for long enough, to find them.

"Suppose I tell you there's a cool thing happening in Houston right now," Kanodia said during his NASA talk. "I do not tell you where it is. I do not tell you when it is happening. I do not tell you what it is. Is it in a bookstore? Is it a music concert? I give you absolutely no priors. It would be a difficult thing to try and find it."

He added: "Houston, we have a problem. We do not know what we're looking for ... and we don't know where to start."

In their study, Kanodia and his colleagues built a mathematical model of what they considered a reasonable-size cosmic haystack.

Their haystack is a sphere of space nearly 33,000 light-years in diameter, centered on Earth. This region captures part of Milky Way's bustling core as well as some giant globular clusters of stars above and below our home galaxy.

They also picked eight dimensions of a search for aliens — factors like signal transmission frequency, bandwidth, power, location, repetition, polarization, and modulation (i.e., complexity) — and defined reasonable limits for each one.

"This leads to a total 8D haystack volume of 6.4 × 10116m5Hz2s/W," the authors wrote.

That is 6.4 followed by 115 zeros — as MIT Technology Review described it, "a space of truly gargantuan proportions."

How much of this haystack have we searched?

Instead of asking, "Where is everybody?" we should be asking, "Why are we barely looking?"

Kanodia and his colleagues then examined the past 60 years' worth of SETI projects and reconciled them against their haystack.

The researchers determined that humanity's collective search for extraterrestrials added up to about 0.00000000000000058% of the haystack's volume.

"This is about a bathtub of water in all of Earth's oceans," Kanodia said. "Or about a five-centimeter-by-five-centimeter patch of land on all of Earth's surface area."

Those numbers make humanity's search efforts seem feeble. But Kanodia views it as an opportunity — especially because modern telescopes are getting better at scanning more objects with greater sensitivity and speed. For example, he said, a 150-minute search this year by the Murchison Widefield Array covered a larger percentage of the haystack than any other SETI project in history.

"That's the purpose of this haystack ... to help better inform future search strategies," Kanodia said.

He also noted that the team's calculations assumed there was only one alien civilization within range of Earth. But more than one may exist relatively nearby.

"In the ocean analogy, we do not have to drain the entire ocean to find a fish," he said. "In the Houston analogy, if there were two cool things, you wouldn't have to look as hard."

Still, there's no guarantee that a figurative fish or needle or cool thing is out there at all.

Another group of scientists, this one at Oxford University, recently took a different approach to the question of aliens. Instead of focusing on the likelihood of finding technosignatures that could be detected, they examined the likelihood that intelligent alien life existed at all.

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